Sunflower (Helianthus annuus) is an annual plant, native to the Americans, that is cultivated for use as a confectionary seed as well as for production of cooking oil. Sunflower is an important agricultural crop in the northern plains of the United States from the Dakotas to the panhandle of Texas, throughout Canada, Argentina, and in Russia, Ukraine, and other European countries.
Verticillium wilt is caused by one of three species of Verticillium fungus, V. dahlia, V. alboatrum, or V. longisporum, which cause significant crop losses among many eudicot plants, including cotton, tomatoes, potatoes, eggplants, peppers, ornamentals, and sunflowers. Monocots, gymnosperms and ferns are immune. Symptoms are superficially similar to Fusarium wilts. Wilt itself is the most common symptom, with wilting of the stem and leaves occurring due to the blockage of the xylem vascular tissues and therefore reduced water and nutrient flow. In small plants and seedlings, Verticillium can quickly kill the plant while in larger, more developed plants the severity can vary. Sometimes only one side of the plant will appear infected because once in the vascular tissues, the disease migrates mostly upward and not as much radially in the stem. (Pegg, G. F., Brady, B. L. (2002) Verticillium Wilts, CABI Publishing, New York, N.Y.). Other symptoms include stunting, chlorosis or yellowing of the leaves, necrosis or tissue death, and defoliation. Internal vascular tissue discoloration might be visible when the stem is cut. No chemical control for the disease is available, but the use of crop rotation and resistant varieties, along with deep plowing, have been found useful in reducing the spread and impact of the disease.
Besides being long lasting in the soil, Verticillium can spread in many ways. The most common way of spreading short distances is through root to root contact within the soil. Roots in natural conditions often have small damage or openings in them that are easily colonized by Verticillium from an infected root nearby. Air borne conidia have been detected and some colonies observed, but mostly the conidia have difficulty developing above ground on healthy plants. (Easton, G. D., Nagle, M. E. and Bailey, D. L. (1969) A method of estimating Verticillium propagules in field soil and irrigation waste water. Phytopathology 59, 1171-1172.) In open channel irrigation, V. dahliae have been found in the irrigation ditches up to a mile from the infected crop. Verticillium wilt begins as a mild, local infection, which over a few years will grow in strength as more virile strains of the fungus develop. If left unchecked the disease will become so widespread that the crop will need to be replaced with resistant varieties or planted with a new crop altogether. (Agrios, George N., Plant Pathology, 5th Edition).
Control of Verticillium can be achieved by planting disease free plants in uncontaminated soil, planting resistant varieties, and refraining from planting susceptible crops in areas that have been used repeatedly for solanaceous crops. Soil fumigation can also be used, but is generally too expensive over large areas.
In tomato plants the presence of ethylene during the initial stages of infection inhibits disease development, while in later stages of disease development the same hormone will cause greater wilt. Tomato plants are available that have been engineered with resistant genes that will tolerate the fungus in their system, while showing significantly lower signs of wilting.
Verticillium albo-altrum, Verticilium dahliae and V. longisporum can overwinter as melanized mycelium or microsclerotia within live vegetation or plant debris. As a result it can be important to clear plant debris to lower the spread of disease. Verticillium dahliae and V. longisporum are able to survive as microsclerotia in soil up to 15 years.
Verticillium Wilt caused by the soil borne fungus Verticillium dahliae, is one of the most significant diseases in Argentina and is a growing problem in the United States. One of the major control strategies for this disease is to plant resistant varieties. A single dominant gene is believed to control Verticillium wilt resistance in sunflower (Fick and Zimmer 1974).